Electrorheological materials are fluid compositions which exhibit substantial changes in rheological properties in the presence of an electric field. Electrorheological materials typically consist of (1) a carrier fluid, (2) a particle component, (3) an activator, and (4) a surfactant. The surfactant of the electrorheological material is utilized to disperse the particle component within the carrier fluid while the activator is utilized to impart electroactivity to the particle component. In the presence of an electric field, the particle component becomes organized so as to increase the apparent viscosity or flow resistance of the overall fluid. Therefore, by manipulating the electric field, one can selectively change the apparent viscosity or flow resistance of an electrorheological material to achieve desired results in various known devices and applications.
In the absence of an electric field, electrorheological materials exhibit approximately Newtonian behavior; specifically, their shear stress (applied force per unit area) is directly proportional to the shear rate (relative velocity per unit thickness). When an electric field is applied, a yield stress phenomenon appears and no shearing takes place until the shear stress exceeds a yield value which rises with increasing electric field strength. This phenomenon can appear as an increase in apparent viscosity of several, and indeed many, order of magnitude.
The mechanism responsible for the observed behavior of electrorheological materials is believed to be an induced polarization of the particle component (particles) followed by a mutual interaction of the polarized particles to form a filamentary structure. In general, the particles in an electrorheological material are able to polarize due to internal or surface conductivity which leads to Maxwell-Wagner polarization when an external field is applied. Although polarization can also occur due to electronic or atomic distortions and the orientation of molecular dipoles, i.e. the real part of the dielectric constant, conduction and subsequent Maxwell-Wagner polarization will dominate at low frequency.
Induced polarization in most electrorheological materials, particularly the so called "water-activated" materials is due to ionic conduction. Adsorbed water on the surface of these particles form an electrolyte with Ca or an alkali metal such as Na, K or Li which are generally present as impurities or are added on purpose to form mobile cations. These cations move through the pores and along the surface of the particles under the influence of an external field to form induced dipoles. An activator such as water is required by these electrorheological materials in order to solvate the cations. If the activator is removed, the ions are no longer mobile and polarization can no longer occur or occurs so slowly that little electrorheological effect is observed. The activator for these materials can also be solvents or molecules containing an amine or an alcohol functionality such as ethylene glycol, diethylamine or acetamide such as is discussed in U.S. Pat. No. 3,427,247 and Matsepuro, "Structure Formation in an Electric Field and the Composition of Electrorheological Suspensions," Royal Aircraft Establishment Library Translation 2110, July 1983.
For electrorheological materials in general, a higher volume fraction of particle component affords a higher induced yield stress and the relationship between induced yield stress and volume fraction has been found to be approximately linear for volume fractions up to about 50%. Volume fractions greater than 50% are generally not used since the materials become very strongly dilatant above this point. Above a 50% volume fraction the zero-field viscosity and zero-field yield stress increases so rapidly that the proportional change in stress due to the applied electric field is actually less than that obtained for a volume fraction less than 50%.
Particle size has little influence on the magnitude of the electrorheological effect as long as the particles have a diameter more or less within the range of 0.1 to 100 microns. Particles smaller than this range may show a decreased effect due to competition from thermal effects, e.g. Brownian motion, which tends to inhibit formation of particle chains when the electric field induced particle-particle interaction energy is less than or on the same order as the thermal energy kT/2. Particles larger than the above range will continue to exhibit an electrorheological effect; however, they become increasingly difficult to maintain in suspension and are subject to jamming and filter cake packing, i.e. the particles chain but the continuous phase liquid continues to move between them. These effects are minimized by keeping the particle small enough such that the Stokes drag forces experienced by a particle are of the same order as the electric field induced forces.
At a fixed electric field strength, the shear stress of electrorheological materials generally increases linearly with shear rate. The rate of stress increase with increasing shear rate is the plastic viscosity of the electrorheological material. The plastic viscosity is, in general, equal to the zero-field or Newtonian viscosity of the electrorheological material.
Many different types of specific electrorheological materials have been previously developed in an attempt to optimize the parameters and properties discussed above. For example, an electrorheological material utilizing silica gel as the particle component and electrically stable dielectric oily vehicles such as white oils and transformer oils as the carrier fluid is disclosed in U.S. Pat. No. 2,661,596. Water is used as the activator while various dispersing agents such as sorbitol sesquioleate, ferrous oleate, sodium oleate, and sodium naphthenate are utilized as surfactants. Similarly, U.S. Pat. No. 2,661,825 discloses an electrorheological material which utilizes carbonile iron powder or silica gel as the particle component and mineral oil or kerosene as the carrier fluid. Various activators mentioned include water, ethylene glycol, and mono ethyl ether while surfactants utilized include aluminum stearates, lithium stearate, lithium rasinoleate, sorbitol sesquioleate, and lauryl peridinium chloride.
An electrorheological material composed of a non-conductive solid particle component dispersed within an oleaginous carrier fluid is described in U.S. Pat. No. 3,047,507. The compositions utilize as an activator a minimum amount of water and utilize as a surfactant various anionic and cationic surface active agents such as fatty acids, naphthenic acids, resinic acids, various salts of these acids, and primary amines. Also, U.S. Pat. No. 3,367,872 discloses an electrorheological material which utilizes alumina or silica alumina as the particle component and an oleaginous vehicle as the carrier fluid. Water is described as the activator and various anionic and cationic agents such as alkyl aryl sulfonates, sulfated alcohols, oleyl alcohol sulfates, lauryl alcohol sulfates, various sodium alkyl sulfates, quaternary ammonium salts, and salts of higher alkyl amines are described as surfactants.
Traditional electrorheological materials such as the materials described above require both a particle component and a surfactant in order to perform effectively in various applications. It would be desirable to eliminate the need for both a particle component and a surfactant in present electrorheological materials.
Turning to more specific applications, in order to fulfill their potential as a unique interface between electronic controls and mechanical systems, appropriate electrorheological materials must demonstrate certain practical characteristics. For example, in certain applications an electrorheological material should be miscible with water to facilitate handling of the material and cleaning of mechanical systems containing the material. Also, in applications involving mechanical components or objects having delicate surfaces, the dispersed phase particles should be non-abrasive. As would be expected, the chemical nature of the carrier fluid, the particle component, and any resulting combination should be compatible with the mechanical materials used to produce the electrorheological device.
One particular group of applications in which it is desirable that electrorheological materials exhibit miscibility with water are fixturing and chucking applications in which electrorheological materials are used to hold or secure an object firmly in place so that it may be machined, measured, gauged or otherwise inspected. Examples of such electrorheological material-based chucking devices are disclosed in U.S. Pat. Nos. 3,197,682 and 3,253,200. One problematic aspect of such devices is that the object to be held is placed in contact with the electrorheological material and after the chucking process is complete an undesirable residue of electrorheological material remains on the surface of the object. This residue is generally oily in nature and may often be pigmented depending on the nature of the dispersed phase. Cleaning of the object after the chucking process is a problem with normal electrorheological materials such as silicates in silicone oil or pigmented fluids. Any advantage incurred by the electrorheological material chucking device may be lost due to the additional time required to clean the part.
It is also important to utilize a non-abrasive particle component in such chucking device applications as well as in other applications such as clutching devices in order to avoid scratching or marring of any object or component surface. Non-abrasive dispersed phase particles are particularly desirable in chucking applications involving parts having a delicate surface finish.
Therefore, it would be desirable to create electrorheological materials which are miscible with water and yet which are physically, mechanically, and chemically compatible with applied systems.